Magnetic field assembly for electro-mechanical transducers



NOV. 26, 1968 c SLOAN 3,413,579 I MAGNETIC FIELD ASSEMBLY FORELECTROMECHANICAL TRANSDUCERS Filed March l4, 1966 FIG.2A.

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DEMAGNETIZATION FORCE H. KILO OERSTEDS wmussses: v INVENTOR @OMMQQCTCarroll D. Sloan 714M W M ATTORNEY United States Patent Office 3,413,579Patented Nov. 26, 1968 3,413,579 MAGNETIC FIELD ASSEMBLY FOR ELECTRO-MECHANICAL TRANSDUCERS Carroll D. Sloan, Murrysville, Pa., assignor toWestinghouse Electric Corporation, Pittsburgh, Pa., a corporation ofPennsylvania Filed Mar. 14, 1966, Ser. No. 534,133 7 Claims. (Cl.335-231) ABSTRACT OF THE DISCLOSURE This invention relates to magneticfield assemblies for electromechanical transducers such as aloudspeakers, and more particularly to magnetic field structuresemploying ceramic or ferrite magnets and to means for securing theelements of such assemblies together.

The use of ferrite or ceramic magnets has been found to have severaladvantages over other magnets, such as those made from the Alnicoalloys, that are customarily used in loudspeakers. First, the use offerrite magnets has been found to be less costly epsecially where thelarger, high powered sizes of magnets are needed. See ondly, ceramicmagnets made of such materials as strontium or barium ferrite typicallyhave high values of demagnetizing force which allows the use of muchshorter lengths of this material than is normal for other typicalloudspeaker magnetic materials. However, greater crosssectional areas ofthe magnet are required due to the comparatively low values of thesimultaneous magnetic induction associated with these ceramic materials.As a result, the ceramic ferrite magnets are typically of a shorteraxial length and the loudspeakers in which they are incorporated may bedesigned to have a significantly shallower depth.

In order to take advantage of the properties of the ferrite magnets,ring type magnetic assembly includes an annularly shaped ceramic magnetdisposed between a back pole plate and a front pole plate having anopening therein. Further, a center pole piece extends from the back poleplate through the opening of the ceramic magnet to form a circular airgap between the inner periphery of the opening of the upper poleplate'and the center pole piece.

However, difliculty has been encountered in firmly securing together thefront and back pole plates to the ferrite magnet. One method of holdingthis assembly together is to place an epoxy adhesive between the facesof the ceramic magnet and the corresponding faces of the front and backpole plates, but this process has been found to be very expensive due tothe excessive fixturing and curing times required, the cost of theadhesive, and the necessary steps of preparatory cleaning. Further, itis diificult, if not impossible, to reclaim the magnets from theseassemblies after they have been secured withsuch adhesives. Anotherknown method of securing the magnetic pole plates together is to usefastening means such as screws or rivets. The screws or rivets are ofthemselves relatively expensive to manufacture, and in addition thesefasteners require the expensive steps of punching (or drilling) andtapping the holes in the pole plates, and additional assemblyoperations. A still further method requires the use of a ferrule whichis .a new and improved magnetic assembly for ferrite magnets which iseasily and inexpensively assembled.

'It is a still further object of this invention to provide a new andimproved magnetic assembly for ferrite magnets which may be assembledwithout additional fastening means in a single step as by welding.

It is a still further object of this invention to provide a new andimproved magnetic assembly for ferrite magnets wherein the ferritemagnet may be easily aligned with the remaining elements of theassembly.

Briefly, the present invention accomplishes the above mentioned objectsby providing a magnetic assembly for an electro-mechanical transducersuch as a ioudspeaker which includes front and back pole plates, anannular ferrite magnet disposed between the front and back pole plates,and a central pole piece magnetically associated with the back poleplate and extending into an aperture within the front pole plate toform, a circular air gap between the central pole piece and the frontpole plate. More particularly, the front and back pole plates aresecured together by means which include one or more projections whichare formed as by punching in the front and/ or back pole plates andextend collectively a distance approximately equal to the thickness ofthe ferrite magnet. The front and back pole plates are so disposed withrespect to each other and the protrusions are aligned with theprotrusions of the other plate so that they may be easily weldedtogether. Further, the projections may be disposed adjacent the aperturewithin the front pole plate and the pole piece so as to provide meansfor align ing the annularly shaped ferrite magnet which may be disposedabout these projections.

These and other objects and advantages of the present invention willbecome more apparent when considered in view of the following detaileddescription and drawings, in which:

FIGURE 1 is a sectioned view of an electro-mechanical transducerincluding a magnetic assembly in accordance with the teachings of thisinvention;

FIG. 2A is a plan view taken along the line II-II of FIG. 1 of themagnetic assembly as shown in FIG. 1;

FIG. 2B is a plan view of an alternative embodiment of the magneticassembly which may be incorporated in the electrounechanical transducerof FIG. 1;

FIG. 3 is an elevational view showing the elements of the magneticassembly in accordance 'with this invention, a basket member and theplacement of electrodes during a welding operation, which in accordancewith the teachings of this invention may be accomplished in a singlestep; and

FIG. 4 shows in graph form the demagnetization curve of the magnet to beincorporated within the magnetic assembly of FIG. 1.

Referring now to the drawings and more particularly to FIG. 1, anelectro-mechanical transducer such as a loudspeaker 10 is shown having amagnetic assembly 12 and a basket member 20 secured thereto forsupporting a diaphragm 22. The magnetic assembly 12 includes a frontpole plate 14 and a back pole plate 16 between which there is disposed aceramic, ferrite magnet 18. Typically, the back and front pole plates 16and 14 are made of a suitable flux conducting material such as a lowcarbon steel in order to improve their permeability, although othertypes of ferrous material could be used. The back pole plate 16 isformed with a central, circular opening 26 to securely receive as byforce lit a center pole piece 28. The center pole piece 28 has a neckportion 30 which extends into and is so dimensioned so as to be securedwithin the aperture 26, and an enlarged portion 32 which extends fromthe surface of the back pole plate 16 into a circular aperture 24centrally disposed within the front pole plate 14. Illustratively, thecenter pole piece 28 is of a circular configuration and has a diameterslightly smaller than the diameter of the aperture 24. Further, theaperture 24 is disposed concentrically about the center pole piece toform an annularly shaped air gap 34 therebetween in which a voice coil56 to be hereinafter described can be inserted.

Magnetic flux is provided by the ferrite magnet 18 which is disposedcentrally within an opening 58 of the front pole plate 14. The magnet 18is formed substantially in the same shape as the front and back poleplates 14 and 16 which can be rectangular, circular or any other desiredshape. In accordance with the teachings of this invention, the magneticassembly 12 and the means for securing the elements of this assembly aredesigned to incorporate a ferrite magnet therein. More specifically, theceramic, ferrite magnets may be made of suitable materials such asbarium ferrite (Ba-6Fe O strontium ferrite (SrO-6Fe O and lead ferrite(PbO-6Fe O In one illustrative application of this invention, astrontium ferrite as manufactured under the name Westro-Alpha (atrademark of the Westinghouse Electric Corporation) could be thematerial of which the magnet 18 is made. A demagnetization curve for atypical sample of the magnetic material Westro-Alpha is presented inFIG. 4 to which reference will be made in explaining the advantages ofthese ferrite, ceramic materials.

In order to determine the dimensions of the magnet 18, a value of thedemagnetizing force H is selected upon a demagnetization curve such asshown in FIG. 4. Typically, it is desired to select such values of thedemagnetizi-ng force H to be of a maximum value while excluding thatportion of the curve in which variances may occur due to temperaturechanges. Illustratively, an operating point for the value of thedemagnetizing force H, of approximately 1500 oersteds may be selectedupon the demagnetization curve of FIG. 4. The axial length (shown inFIG. 1 as L of the magnet 18 may be determined from the followingexpression:

Where L =the length of the air gap (i.e. the distance between the poleplate 14 and the center pole piece 28), H is the magnetic fieldintensity within the air gap, and R is a reluctance factor and typicallyhas a value of between 1.1 and 1.3. Further, the cross-sectional arearequired of the magnet 18 may be determined from the expression:

H,,A,K Am- Bd where K equals a leakage factor, B, is the simultaneousinduction value at H,; on the demagnetization curve (see FIG. 4), and Ais the area of the air gap. Thus, where the length of the air gap lengthL is determined to be about .050 inch, the reluctance factor is selectedto be approximately 1.2, and the necessary magnetic field H in the airgap is selected to be approximately 10,000 oersteds, the axial length ofthe magnet L is illustratively determined to be .40 inch. Thecross-sectional area of the magnet 18 may be determined for theillustrative case in which the leakage factor K is determined to beapproximately 2, the area of the air gap is approximately .785 squareinch where the aperture 24 is one inch in diameter, the thickness of thefront plate 14 is .25 inch, and the simultaneous induction value E isdetermined from the curve shown in FIG. 4 to be 2.4 kilogausses; thusthe value of the crosssectional area of the magnet 18 may be calculatedto be 6.5 square inches. Thus, from this illustrative example, it may beseen that high values of the demagnetizing force H, of ferrite magnetsmake possible magnets with substantially shorter lengths than is normalfor other dimensions of commonly used permanent magnet materials. Incomparison, commonly used metallic permanent magnet materials such asAlnico V have a useful value of H, of about 550 oersteds and a value ofB of about 10 kilogausses. Similarly, as may be seen from FIG. 4, thevalues of the simultaneous induction B for ferrite materials are lowerand thus greater areas of the cross-section of the magnet are required.As a result of these dimensional requirements, it is desirable toincorporate ferrite magnets into assemblies providing pole pieces whichtake advantage of the short axial length of the magnet and whichadditionally provide means for directing the flux from substantialcross-sectional areas across the air gap.

Such a design is shown in FIG. 1 where the magnet 18 has an axial lengthL which is relatively small in comparison with its cross-sectionaldimensions. Likewise, the cross-sectional area of the magnet 18 isrelatively large and is effectively used as by the front and back poleplates 14 and 16 which are disposed on either side to direct the fluxtherefrom. Specifically, the magnetic circuit directs the flux from themagnet 18 in the following manner: a first leg of the magnetic circuitis formed by the back pole plate 16 which directs the flux from themagnet 18 therethrough and into the center pole piece 28. The flux isthereby directed across the air gap 24 into the front pole plate 14 tobe returned to the magnet 18. In this manner, the particular propertiesof the ferrite, ceramic magnets may be effectively utilized.

In accordance with the teachings of this invention, the elements of themagnetic assembly 12 may be secured together to position a ferritemagnet in the following manner. The front pole plate 14 is formed withone or more projections 38 therein as by punching which proves to be aninexpensive operation. In a like manner, projections 36 may be formed inthe back pole plate 16. As is shown in FIG. 1, the projections 36 and 38are formed in those portions respectively of the plates 16 and 14 thatare adjacent to the apertures 26 and 24 respectively. Further, theprojections 36 and 38 are so disposed that they are substantiallyaligned with each other. In addition, the projections 36 and 38 are soplaced upon the pole plates 14 and 16 that the magnet 18 may beaccurately and quickly aligned with respect to the openings within thefront and back pole plates 14 and 16. More specifically, the spacings ofthe projections 36 and 38 from the centers of the back plates 14 and 16are so determined, that the magnet 18 may be slipped over either theprojections 38 or 36 and be positioned concentrically with respect tothe openings 24 and 58. Further, the thickness of the front and backplates 14 and 16 is so selected to provide projections 36 and 38 withthe appropriate dimensions. The combined dimensions of the projections36 and 38 are selected to be slightly in excess of the axial length L ofthe magnet 18. In an illustrative embodiment, each of the projections 36and 38 is determined to be in slightly excess of one half of the axiallength L of the magnet 18. Thus, when the projections 36 and 38 arewelded together as will be explained in greater detail later, theprotrusions 36 and 38 upon cooling will shrink so that their combineddimensions will be slightly less than the axial dimension L therebyproviding a tight fit of the magnet 18 between the pole plates 14 and16.

If the magnetic field assembly 12 is to be used as in a loudspeaker, asillustrated in FIG. 1, the basket member 20 may be mounted upon thefront pole piece 14. The conically shaped diaphragm 22 is mounted uponthe basket member 20 in a manner well known in the art and includes thevoice coil 56 composed of windings of wire which are disposed upon thediaphragm 22. Further, the voice coil 56 is disposed within the air gap34 between the enlarged portion 32 of the center pole piece 28 and theinner periphery of the aperture 24 of the front pole piece 14. In orderto secure the basket member 20 to the front pole piece 14, a pluralityof projections 40 may be formed by punching within the basket member 20.The projections 40 are offset from the axis of the basket member so thatthe basket member 20 may be aligned with the front plate 14, and theprojections 40 will abut a portion of the front pole plate 14 adjacentthe depressions associated with the projections 38.

Referring now to FIGS. 1 and 2A, it is desirable for the diameter of themagnet 18 to exceed the cross-sectional dimension of the front and backpole plates 14 and 16. In order to save the material used to make theseplates, the peripheral edge of the magnet 18 should extend beyond thefront and back pole plates 14 and 16 as shown particularly in FIG. 2A.By so disposing the magnet 18, the magnetic flux leakage between thefront pole plate and the back pole plate may be reduced therebyproviding a more efficient magnetic circuit and effecting a saving inthe material of which the pole plates are made.

Referring now to FIG. 2B, an alternative method of securing the frontand back pole plates is shown. Specifically, a front pole plate 14a maybe secured to the back pole plate (not shown) by means which include apair of ears 42 which extend from either side of the front pole plate14a and have impressed therein a projection 38a. It may be understoodthat the bottom pole plate will have an aligned pair of ears andprojections to coincide with those of the front pole plate 14a. In amanner to be described, the projections may then be welded together tothereby secure the front and back pole plates. It may be understood thata magnet is disposed between the pole plates and that the projections38a extend beyond the periphery of the of the magnet.

Referring now to FIG. 3, there is shown an illustrative method ofwelding the front and back pole plates 14 and 16 respectively to eachother. First, the central pole piece 28 is assembled with the back poleplate 16 as by press fitting the portion 30 within the aperture 26 ofthe plate 16. Next, the opening 58 within the magnet 18 is disposedabout either of the sets of projections 36 or 38 to thereby align themagnet 18 with respect to the pole plates. The other pole plate isbrought into position so that the projections 38 and 36 of each of thepole plates are aligned with each other. Next, the basket member 20 ismountedupon the front pole plate 14 so that the projections 40 closelyabut the surface of the pole plate near the depressions associated withthe projections 38. Then, a pair of electrodes 46 and 48 havingrespective contact points 50 and 52 are brought into position so thatthe contact points 50 closely abut a portion of the basket member 20close to the projections 40 and so that the contact points 52 closelyabut the depressions associated with the projections 36 of the back poleplate-16. Finally, voltage is then applied between the electrodes 46 and48 to thereby provide a current flow generally indicated by the numeral54 through the front pole plate 14 and its projection 38, the back poleplate 16 and its projections 36, and the basket member 20 and itsassociated projections 40.

Further, a shielding member or means 60 may be assembled about the polepiece 28 and between the pole plates to prevent the small particlesresulting from the welding of the projections 36 and 38 from beingattracted into the air gap 34 to thereby effect the performance of theloudspeaker. More specifically the shielding member 60 may be of acylindrical configuration and made of a suitable insulating materialsuch as reinforced paper or other fibrous material.

Further, it is noted that the ferrite material of which the magnet 18 ismade has a very high resistivity and thereby the current flow isprimarily restricted to a path through the projections 38 and 36. On theother hand, other typical magnetic materials such as Alnico are highlyconductive and such a material would short the current therebypreventing the use of such a welding technique. In

particular, typical ceramic magnetic materials such as the strontiumferrite marketed under the trademark Westro- Alpha has a resistivity inthe order of 10 ohms per cubic centimeter. Thus, the magnetic assembly12 is capable of being assembled and secured together in a single stepinvolving the welding of the front and back pole plates together.

As mentioned above, the magnetic path of the flux is directed throughthe bottom pole plate 16, the center pole piece 28, across the air gap34 and back to the magnet 18 through the top pole plate 14. As shown inFIG. 1, there would be a tendency for the projetcions 36 and 38 to shortthe magnetic path across the magnet 18-. However, the area of contactbetween the projections 36 and 38 should not exceed about 2.0% of theentire crosssectional area presented between the pole plates and themagnet 18. This small shunting area readily saturates and with such asmall percentage of the magnetic flux being shunted across the magnet18, the magnetic sub-assembly 12 does provide an efficient magneticcircuit to direct flux across the air gap and the voice coil. Further,the contact area should be large enough so that a strong weld iseffected between the projections 36 and 38.

Thus, there has been shown an improved magnetic assembly forelectro-mechanical transducers and in particular for loudspeakers whichprovides an improved means of securing the parts of the assemblytogether. The structure of the magnetic assembly is particularly adaptedto use with the ferrite magnetic materials which due to their inherentmagnetic characteristics are adapted to a design in which the axialdimensions of the magnet is shallow in comparison with thecross-sectional area exposed to the magnetic assembly. Thus, the meansfor securing the magnetic structure of this invention effectivelysecures a pair of pole plates in an inexpensive manner as by weldingwithout substantially impairing the magnetic characteristics of thisassembly.

Since numerous changes may be made in the above apparatus and differentembodiments of the invention may be made without departing from thespirit thereof, it is intended that all matter contained in theforegoing description or shown in the accompanying drawings, shall beinterpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. A magnetic field structure for an electromechanical transducercomprising first and second spaced magnetic plates, at least one of saidfirst and second flux conducting plates having projection meansextending therefrom, ferrite magnetic means disposed between and in aflux transferring relationship with said first and second fluxconducting plates, said first and second flux conducting plates beingsecured together as by said projection means, said projection meansbeing welded to the opposite flux conducting plate from which saidprojection means extends.

2. A magnetic field structure as claimed in claim 1, wherein saidferrite magnetic means is associated with said first and second fluxconducting plates to provide an area of flux transfer, said projectionmeans presenting a contact area to said opposite flux conducting platewhich does not exceed about 2.0% of said area of flux transfer.

3. A magnetic field structure as claimed in claim 1, wherein said fluxconducting plate has an opening therein; and a magnetic pole piece isassociated with said second flux conducting plate so that magnetic fluxmay be transferred therebetween, and extends into said opening in aspaced relation with said opening to provide an air wherein theperipheral edge of said ferrite magnetic means extends beyond theperipheral edges of said first and second flux conducting plates tothereby reduce the flux leakage between said first and second fluxconducting plates.

6. A magnetic field structure as claimed in claim 1, wherein saidprojection means include first and second projection members extendingrespectively from said first and second flux conducting plates, saidfirst and second projection members having a combined first dimension,said ferrite magnetic means having a width of a second dimension, saidfirst dimension being less than said second dimension so that when saidfirst and second projection members are welded together that saidferrite magnetic means is firmly secured between said first and sec- 15ond pole plates.

7. A magnetic field structure as claimed in claim 3, further including ashielding means disposed between said projection means and said air gapto prevent particles resulting from the welding of said projection meansfrom 5 being drawn to said air gap.

FOREIGN PATENTS 521,809 5/1940 Great Britain. 920,232 3/1963 GreatBritain.

GEORGE HARRIS, Primary Examiner.

